Factors affecting persistence of Listeria monocytogenes need to be identified for evaluation and prioritization of interventions.

The foodborne pathogen Listeria monocytogenes can survive over time in food processing environments such as produce facilities. These “resident” Listeria strains increase the likelihood for finished product contamination, recalls, and outbreaks. Advances in sequencing allow for enhanced discrimination between Listeria strains, such as those that may be unique to a specific facility. This results in improved traceback from listeriosis patients to the facility where the implicated food was prepared. However, sequencing advances have also enabled us to better understand how a unique Listeria strain may survive and spread in a facility over time. Thus far, there is little peer-reviewed research on how to prevent, eliminate, or manage a “resident” Listeria strain in a facility, particularly for the produce industry. This project was designed to review published and unpublished data to identify factors that may contribute to a Listeria persisting in a facility and to validate potential interventions suitable for produce facilities using experiments in commercial facilities as well as computer modeling. This project will provide industry with tools to (i) help identify what characteristics of their facilities may allow a Listeria to persist, and (ii) select and justify interventions that are used to prevent, eliminate, or manage Listeria persistence.

Persistence of Listeria in produce packing and fresh-cut facilities continues to be a concern that the industry tries to address through development and implementation of “seek and destroy” programs. While industry has considerably improved their ability to detect Listeria persistence (“seek”), identification and implementation of strategies to eliminate or manage persistence (“destroy”) remains a major challenge. There thus is a need for the produce industry to have improved resources to (i) rapidly identify factors (root causes) that may be responsible for or contribute to Listeria persistence, as well as factors that contribute to dispersal of resident strains; and (ii) identify appropriate science-based interventions that can be used to prevent, eliminate, or manage relevant root causes. In particular, the produce industry has a need for data and procedures to validate Listeria persistence interventions, which will allow produce facilities to justify a given control strategy to regulatory agencies, customers, and third-party auditors. This project was thus designed to (i) assemble a comprehensive list of factors that contribute to the establishment and dispersal of “resident” Listeria in produce packing and fresh-cut facilities and to (ii) use different approaches to validate interventions that target these factors. Validation approaches will include (i) identifying relevant previous scientific literature; (ii) experimental validation in commercial facilities, and (iii) in silico validation utilizing agent-based models for produce packinghouses and fresh-cut facilities. To achieve these project goals, we propose the following objectives: Obj. 1: Conduct a systematic review of published and unpublished data and literature to identify modifiable factors that may contribute to resident Listeria in produce packing and fresh-cut facilities and relevant interventions, assess the validity of these findings based on the strength of evidence and prioritize interventions for assessment in Objs. 2 and 3 using expert elicitation. Obj. 2: Use controlled experiments and observational studies to validate selected interventions identified in Obj. 1 in produce packing and fresh-cut facilities with resident Listeria. Obj. 3: Validate selected interventions that are challenging to validate experimentally (e.g., extensive facility modifications) using our previously developed agent-based model. Results from all three objectives will be used to assemble a resource document that includes (i) a comprehensive list of factors that are well supported to contribute to Listeria persistence and dispersal of resident strains, and (ii) interventions for each factor, along with justification and data supporting each intervention. In addition, we will also develop a step-by-step approach for conducting a root cause analysis to identify the most likely factors contributing to a given persistence event and selecting appropriate interventions. These resource documents will be assembled into a toolkit and made available through different mechanisms, including collaborations with produce trade organizations, to facilitate their widespread use. Ultimately, the project outcomes will allow industry to more effectively control Listeria persistence, leading to a lower risk of cross-contamination and recalls.

1. Conduct a systematic review of published and unpublished data and literature to identify modifiable factors that may contribute to resident Listeria in produce packing and fresh-cut facilities and relevant interventions, assess the validity of these findings based on the strength of evidence and prioritize interventions for assessment in Objectives 2 and 3 using expert elicitation. 

2. Use controlled experiments and observational studies to validate selected interventions identified in Objective 1 in produce packing and fresh-cut facilities with resident Listeria. 

3. Validate selected interventions that are challenging to validate experimentally (e.g., extensive facility modifications) using our previously developed agent-based model.

See this project's final report PDF for tables referenced here. See Table 2.5 for a summary of the summary and observations of the learnings of the current study. Common sites of persistent Listeria from the rapid review (objective 1) and sites of repeat isolation from the longitudinal study (objective 2) include floor drains, floor wall junctures (including floors and walls in separate samples), forklifts, forklift stops, tables, slicers, wax area equipment frames, catch pans, PVC pipes, among other sites. Industry should consider these sites in their own routine and for-cause investigation sampling plans when a lack of Listeria control is identified. Academia should also include similar sites in future studies of persistence in produce operations and should consider studies that develop additional intervention strategies to eliminate persistent Listeria from these sites. Monthly cleaning and sanitation of forklifts, which utilizes foamers for application of cleaning and sanitation chemicals, appears to be effective at controlling Listeria on forklifts. However, testing in each individual operation is recommended to confirm a monthly frequency is truly sufficient due to differences from operation to operation. Cleaning and sanitation protocols that involve breaking down equipment to expose potential niches appear to be effective for eliminating persistent Listeria or repeat isolation of Listeria, based on the rapid review and the longitudinal study. Alternative sanitizer chemistries may be more effective than one another; however, identifying locations on a piece of equipment that are difficult to reach may be more important for sufficient cleaning and sanitation. Additional research is needed to confirm this hypothesis. Agent-based models provide an alternative strategy for testing resource intensive interventions for eliminating persistent Listeria from produce operations. However, modifications made to the models to allow for tracking persistence require large amounts of computing power. Additional studies are needed that assess Listeria persistent in produce operations, and universal definitions of persistence should be developed to facilitate comparison of findings across studies. These areas represent topics that should be considered in future studies.